Thermoplastic polyester resins, for example, polyalkylene terephthalate resins, represented by polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) resins, have conventionally been employed as an engineering plastics material to form a variety of useful articles, such as synthetic fibers, films, sheets and molded components that are used in a number of industrial activities owing to the desirable processability as well as the mechanical, electrical, physical and chemical properties that such resins possess. Continual property improvements are needed, however, as new end-use applications are identified which demand new and/or enhanced resin properties.
In this regard, polycyclohexylenedimethylene terephthalate (PCT) resin (which is a polycondensation reaction product using 1,4-cyclohexanedimethanol as the alkylene glycol comonomer) has attracted considerable attention as a candidate material to replace PET and PBT in at least some recently identified end-use applications, particularly in view of the greater thermal resistance properties which PCT resin exhibits as compared to PET and PBT resins. That is, since PCT resins typically have melting points as high as between 290.degree. C. to 310.degree. C., they are hopeful candidates for those end-use applications which require high temperature resistance properties that are not satisfied by PET and/or PBT resins.
Conventional PCT resins, however, have poor melt-stability properties. That is, conventional PCT resins usually undergo thermal decomposition when processed in the melt phase (e.g., during melt-blending and/or molding operations) due to the fact that a relatively small temperature differential exists between the melting point and the thermal decomposition point for conventional PCT resins generally. This melt-phase deterioration of conventional PCT resins is of sufficient magnitude which precludes high quality moldings from being obtained.
Furthermore, the melt-instability of conventional PCT resins usually results in the formation of normally solid deposits on processing equipment. For example, when conventional PCT resins are melted and formed into films or sheets by extrusion though a slit shaped nozzle with the film or sheet thereafter being cooled by contact with a chill roll, decomposition products (e.g., terephthalic acid) typically adhere to various surfaces of the processing equipment (e.g., the nozzle, chill roll surface, and the like) with which the resin comes into contact. Use of processing equipment which is contaminated with such decomposition products will thus inevitably result in moldings which have unusually rough surfaces and/or reduced gloss characteristics. To prevent this problem, therefore, the processing equipment must be taken out of service and cleaned at periodic intervals resulting in costly production line "down time" (e.g., not only in terms of lost production but also increased labor costs associated with equipment cleaning).
Prior attempts to solve the problems described above have included incorporating various low molecular weight additives, such as antioxidants and catalyst deactivators, into the PCT resin. However, although the thermal stability associated with the resin is improved somewhat by these additives, there are other problems which the additives cause, such as additive "bleeding" and/or reduced surface gloss. Thus, there still exists a need in this art for improved PCT resins which exhibit improved melt-stability. It is towards fulfilling this need that the present invention is directed.
Broadly, the present invention is directed to novel PCT resins which are the copolymerization reaction product of (a) an aromatic dicarboxylic acid or an ester-forming derivative thereof, (b) an aliphatic dihydroxy compound which includes at least 35 mole % of of units derived from 1,4-cyclohexanedimethanol, and (c) a melt-stable effective amount of a specific ester-forming diol compound.
Units derived from the preferred ester-forming diol compound will be present in the molecular structure of the PCT resins of the present invention in an amount between 0.1 to 35 mole %, based on the amount of the aromatic dicarboxylic acid. In this connection, the ester-forming diol compound which is employed in the present invention is a compound of the following formula I: EQU HO--R--O--A--O--R--OH (I)
wherein A is a divalent organic radical having at least one aromatic ring, and R is a divalent organic radical selected from aliphatic hydrocarbon groups having 2 to 8 carbon atoms and polyoxyalkylene groups. Most preferably, A in the formula I represents groups derived from 2,2-bis(4-phenylene) propane, 4,4'-biphenylene, phenylene, 4,4'-diphenylene and naphthylene, while R most preferably represents groups selected from ethylene and isopropylene.
The aromatic PCT copolyester resins according to the present invention are exceptionally stable in the melt phase and thus can be employed to produce molded articles of surprisingly high quality (e.g., in terms of surface smoothness and gloss characteristics). Moreover, these advantageous qualities are achieved without impairing the other desirable properties of PCT resins generally.
Further aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.